Measurement of hardness depth of metals has been previously reported by the American Society for Metals, Metals/Materials Technology Series 8408-003, EFFECTIVE CASE DEPTH MEASUREMENT BY AN ULTRASONIC BACKSCATTER TECHNIQUE, Morris S. Good, 15-20 Sep. 1984, herein incorporated by reference.
This paper reported use of an ultrasonic shear wave technique wherein the ultrasonic transducer was inclined at an angle of 18 degrees from a specimen orthogonal cross sectional plane. The transducer was 13 mm in diameter and had a 5.1 cm focal length, and a frequency of 10 MHz. Signals were generated and received through an ultrasonic pulser/receiver. Received signals passed through an amplifier to a waveform digitizer and averager, then to a CRT display. The CRT display was photographed and measurements of hardness depth taken from the photographs.
The specimen was rotated and 256 signals were obtained around the periphery of the specimen. Signal envelopes were averaged in the waveform digitizer and averager, thereby providing a stable and consistent envelope profile. The averaged envelope profile was used to determine onset of backscatter from the core grain structure. This envelope averaging was compared to metallurgically measured effective hardness depth and found to have a linear relationship therewith. It was reported, however, that the slope of the linear relationship of backscatter arrival time versus Rc 60 effective case depth deviates from the theoretical by as much as 45%.
The paper, specifically FIGS. 5B and 5C showing A-scans or amplitude scans, indicates a distortion of the signal envelopes. Ideally, the radio frequency (RF) signal received from the transducer and converted to a video signal amplitude should produce a smooth, well resolved envelope of the rectified RF signal. Instead, the video signal produced sharp peaks. Averages of signals having sharp peaks produced an envelope (FIG. 5C) having a rough signal with a Jittery profile. In addition, the low amplitude signals in the RF signal are shown as a flat zero line in the video signal, thereby creating an artificial filter that rejects low amplitude signals. The envelope roughness, together with low amplitude signal rejection, reduces the accuracy of a threshold determination for the video signal. Loss of resolution of the envelope peak reduces the accuracy of the slope of the signal line between the low amplitude signals and the peak. Because it is this signal line and its slope that are used in determining hardness depth, it is desirable to obtain a line as accurate as possible.
Another source of surface hardness depth measurement error is seen in the paper in FIGS. 7B and 7C, wherein FIG. 7B the peak is about 4 scale units, and in FIG. 7C the peak is about 5 scale units. In the extreme, the peaks from part to part may be so disparate that it would be very difficult, if not impossible, to set a threshold limit. It is important that a hardness depth measurement system or procedure result in correct determination of hardness depth even when there is a high part to part amplitude variation.
Moreover, the fixture used and shown in FIG. 4 of the paper is rigid and does not account for part surface variation, for example eccentric surface variations. Because variations in surface geometry directly affect the determination of hardness depth, inaccurate measurements would result from a system that does not consider those variations.
Although not stated in the article, the envelope averaging was accomplished with electronic hardware, specifically an A/D converter made by Tektronix, Beaverton, Oregon, and the overall measurement procedure was quite slow, requiring from about 1 to about 3 minutes to obtain the 256 scans around the periphery of the specimen and significant additional time to subsequently determine hardness depth.
The slow measurement procedure coupled with the inaccuracies discussed above, must be overcome in order for such a system to be implemented in a commercial production facility.